Singapore was hit by its first riot in four decades on Sunday, when a crowd of 400 people ran amok through the serene city-state’s Little India neighborhood following a bus crash that claimed the life of a 33-year-old Indian man.

The violence has been deemed a reflection of heightened tensions between the state and an impoverished immigrant labor force. According to local police, 27 people, all of South Asian origin, have been arrested in the resulting commotion.

Young men set about attacking the crashed bus at the junction of Race Course Road and Hampshire Road soon after 9 p.m. local time. Emergency-service personnel trying to remove the body of the victim reportedly found themselves under a hail of missiles. Around 300 officers were sent on to quell the baying crowd, but the unrest quickly escalated and several cars were soon set ablaze. Around 18 people were reportedly injured.

Singapore Prime Minister Lee Hsien Loong took to his Facebook page to decry a “very grave incident” and to promise that the authorities will “spare no effort to identify the culprits and deal with them with the full force of the law.”

The last public discord of this scale was when Chinese and Malay residents clashed in 1969. But according to Bridget Welsh, an associate professor of political science at Singapore Management University, such an eruption had long been brewing, although the “scope was still shocking” considering the spark was a simple traffic accident. “The workers should be able to think that they can go to the police [after the bus crash] and get some sort of justice,” she tells TIME. “So the interesting question is, why didn’t they and why instead they reacted the way they did.”

Singapore, which boasts a population of just 5 million, saw its strongest outbreak of labor unrest last year when 170 bus drivers from mainland China went on strike illegally. “You have a very tense relationship — a difficult working environment and not enough protection from the government,” explains Welsh.

Little India is usually teeming with people on Sundays, when many construction workers from Bangladesh and India gather during their free day from work. But increasingly, the atmosphere in this vibrant district has been combustive. “A lot of foreign labor feels increasingly exploited,” adds Welsh, “and on the other side you have a population that is so angry about all these workers coming in.”

Partial reforms in recent years have sought to improve the lot of immigrant labor, but Singapore’s 200,000-odd foreign domestic workers remain excluded from the Employment Act and key labor protections, like limits on daily work hours.

According to Reuters, Singapore Police Force Commissioner Ng Joo Hee said: “As far as we know now, there was no Singaporean involved in the riot. The unwanted violence, rioting, destruction of property, fighting the police, is not the Singapore way.”